The molecular parameters of tryptophan hydroxylase from rat brainstem and murine mast cell, were determined by gel filtration and sucrose density gradient ultracentrifugation. The enzyme from rat brainstems has a calculated molecular weight of 220,000 daltons, a Stokes radius of 55.6 A, a frictional ratio of 1.28, and a sedimentation coefficient of 9.63S. The mast cell enzyme has a molecular weight of 144,000 daltons, a stokes radius of 50.3 A, a frictional ratio of 1.35, and a sedimentation coefficient of 6.97S. Evidence for catalytically active subunits was not found. It appears that the tryptophan hydroxylase species from rat brainstem and murine mast cell represent distinct molecular entities.